Double-hop transmission is a condition where two ground stations may indirectly communicate with each other with the assistance of an intermediate ground station. A double-hop satellite link typically arises where two ground stations are sufficiently remote from each other that they do not have a direct link to the same satellite. As a result, the signal travels through space twice.
In particular, a transmitting ground station sends the signal through a first satellite to the intermediate ground station. Then, the intermediate ground station sends the signal through a second satellite to a recipient ground station.
Several disadvantages of double-hop satellite links are known. Double-hop satellite links double the satellite delay normally attributed to direct satellite links. Also, double-hop transmission is known for producing an inconvenient delay in sound for video conferencing. In this regard, a user may see parts of a video transmission before the associated sound is played. Furthermore, service costs may be doubled and security problems may arise with the use of an additional satellite and an additional ground station.
One way to reduce double hop delays is to provide an inter-satellite link to allow for secure, high capacity communication channels around the world. One problem with inter-satellite links is that the transmitter power is relatively high. In satellite communications, reducing the power consumption by the various components is typically a goal. One way in which to reduce output power by fifty percent is to employ an encoding scheme known as differential phase shift keying (DPSK). Differential phase shift keying requires a demodulator on the receiver to translate the received data from optical phase encoding to standard logical zeroes and ones. In DPSK, each bit is compared with the previous bit and, if the bit differs from the previous, a phase change of 180 degrees is placed on the optical carrier. If the bit is the same, no change in phase is made. DPSK has an advantage over regular phase shift keying in that there is no need for a local oscillator or phase reference. An optical system, employing a local oscillator as a reference, is a complex, non-trivial problem that is extremely difficult to implement.
Quadrature phase shift keying (QPSK) modulation is a standard format commonly implemented in RF communications. However, due to the short wavelengths inherent in optical communications, phase modulation is generally not used. Most fiber-optic links employ on-off keying and typically ignore phase shift keying. This is due to the tight tolerances imposed by the high frequency of the carrier.
Quadrature phase shift keying allows a doubling of the data throughput without an increase of clock speed. QPSK modulation has four symbols rather than two found in typical binary communications. The four symbols are represented by four distinct phase shifts, each separated by ninety degrees (0°, 90°, 180°, and 270°). In QPSK, each transmitted symbol represents two bits of data (00, 10, 11, or 10). Thus, the actual data rate is double the clock rate. Differential QPSK (D-QPSK) is similar to traditional DPSK in that it compares previous bits with present bits to achieve encoding. No such implementation in the optical industry is known.
It would therefore be desirable to provide a system that increases a throughput of an optical system to make inter-satellite links more viable.